1. Technical Field
The present invention relates to a satellite signal tracking method, a position calculating method, and a position calculating device.
2. Related Art
A global positioning system (GPS) is known as a positioning system using a positioning signal and is used in a position calculating device built in a mobile phone or a car navigation apparatus. The GPS performs a position calculating operation of calculating a position coordinate and a clock error of the position calculating device on the basis of information and the like on positions of plural GPS satellites or pseudo distances from the GPS satellites to the position calculating device.
GPS satellite signals transmitted from the GPS satellites are modulated with spread codes, which are different depending on the GPS satellites and called C/A (Coarse/Acquisition) codes. In order to capture a GPS satellite signal from week reception signals, the position calculating device performs a correlating operation of correlating the reception signals with replica codes which are replicas of the C/A codes and captures the GPS satellite signal on the basis of the correlation values. When the position calculating device once successfully captures the GPS satellite signal, it tracks the captured GPS satellite signal.
The tracking of the GPS satellite signal is carried out by a carrier tracking loop circuit tracking carrier waves (carriers) or a code tracking loop circuit tracking phases of the C/A codes (for example, JP-A-2008-232761).
In a loop circuit, it is known that a noise element coexists in a signal due to various reasons. In order to remove the noise element, the loop circuit is generally provided with a loop filter such as a low-pass filter.
Particularly, resistance to a jitter (an offset of a signal in the time axis direction, a phase variation, or a frequency variation) is almost determined depending on the loop filter. When the loop bandwidth of the loop filter is narrowed, the resistance to the jitter is improved but a part of a signal component may be cut. Accordingly, there is a problem in that a signal tracking property deteriorates and a lock failure of losing a captured GPS satellite signal can be easily caused in some extreme cases. On the contrary, when the loop bandwidth is widened, the resistance to the jitter is lowered and thus the position calculating device may be in a non-lockable state (where “lock” means tracking a GPS satellite signal) in some extreme cases.
In the related art, the value of the loop bandwidth is fixedly determined in consideration of the balance at the time of designing the loop filter and then the loop filter is constructed. However, the jitter introduced thereto can vary due to noise elements such as thermal noise or dynamic stress. Particularly, the dynamic stress can greatly vary depending on the situation of movement of the position calculating device. Accordingly, when the fixed loop bandwidth is used, there may be a problem in that the tracking property of tracking a GPS satellite signal is lowered.